1. Field of the Invention
This invention relates to a process for the production of light-colored washing active .alpha.-sulfofatty acid lower alkyl ester salts by pretreatment of fatty acid lower alkyl esters, reaction with gaseous sulfur trioxide and subsequent working up.
2. Statement of Related Art
The sulfonation of fatty acid lower alkyl esters for the production of .alpha.-sulfofatty acid lower alkyl ester salts (hereinafter referred to as "ester sulfonates") has been known for some time (J.Am.Oil.Chem.Soc. 39, 490 (1962)). On an industrial scale, the starting materials used are fats and/or oils of vegetable or animal origin from which the fatty acid lower alkyl esters are obtained either by lipolysis and subsequent esterification with lower alcohols or by transesterification of the natural triglycerides with lower alcohols. Depending on the origin of the natural raw material, the fatty acid ester mixtures obtained contain esters of C.sub.6-22 fatty acids. Technical fatty acid lower alkyl ester such as these are sulfonated with gaseous sulfur trioxide by known methods which are described, for example, in German patent applications DE-A-12 48 645 and DE-A-11 79 931. This leads to more or less heavily discolored acidic crude products which are bleached and converted into the corresponding .alpha.-sulfofatty acid lower alkyl ester salts by neutralization at pH 6 to 7. In this form, the ester sulfonates have acquired considerable significance as surfactants for the production of detergents and cleaning preparations.
Despite optimal sulfonation conditions, however, dark-colored products are frequently obtained in the production of ester sulfonates and cannot be lightened in color sufficiently, if at all, by conventional bleaching methods. Products regarded as critical in this regard have a Klett color value after bleaching (as determined in a 1 cm round cuvette at a concentration of 5% by weight washing-active substance in water) above 30. Ester sulfonates of this type are unsuitable for the production of detergents and cleaning preparations on aesthetic grounds.
There has been no shortage of attempts in the past to develop processes by which light-colored ester sulfonates could be produced.
According to the teaching of German patent application DE-A-12 46 718, it is essential for the production of light-colored ester sulfonates to use only those fatty acid esters in which the content of unsaturated compounds does not exceed 0.1 to 0.5% by weight for the sulfonation step. In practice, however, it has been found that even fatty acid lower alkyl esters containing 0.1 to 0.3% by weight unsaturated compounds can give rise to ester sulfonates of unsatisfactory color quality.
According to DE-C-12 62 265, mixtures of fatty acid lower alkyl esters and at least 25% by weight of an alkyl benzene are together reacted with sulfur trioxide with a view to improving color quality. However, it is only possible by this method to obtain mixtures of anionic surfactants which contain alkyl benzenesulfonates in addition to ester sulfonates.
DE-A-14 43 995 shows that ester sulfonates of good color quality can be obtained by limiting the degree of sulfonation to around 90%. However, ester sulfonates having a low degree of sulfonation lead to problems in the manufacture of powder-form detergents by spray-drying because experience has shown that the processing of such products is accompanied by intensive pluming.
Finally, EP-B-0 054 724 describes a process for the production of light-colored ester sulfonates in which fatty acid esters of which the fatty acid glyceride content has been reduced by distillation to values of 0.3 to 0.5% by weight are used for the sulfonation step. In practice, however, it has been found that this measure is not in itself sufficient to guarantee the production of light-colored ester sulfonates irrespective of the starting material and its pretreatment.
Accordingly, the problem addressed by the invention was to develop an improved process for the production of light-colored washing-active .alpha.-sulfofatty acid lower alkyl esters which would not be attended by any of the abovementioned disadvantages.